The present invention relates to an apparatus and a method for controlling power converters in a DC power transmission system, particularly for controlling active power preferential converters in which an automatic power controller (APC) or automatic frequency controller (AFC) and an automatic reactive power controller (AQC) or automatic voltage controller (AVC) are employed.
FIG. 1 shows a conventional power converter control apparatus for a DC power transmission system. Details of each element shown in FIG. 1 are known to a skilled person in the art. In FIG. 1, the DC circuit of a converter 1A is coupled via a DC reactor 2A, DC power transmission lines 3 and a DC reactor 2B to the DC circuit of a converter 1B. The AC circuit of converter 1A is coupled via a converter transformer 4A and a circuit breaker 5A to a 3-phase AC power line 6A. The AC circuit of converter 1B is coupled via a converter transformer 4B and a circuit breaker 5B to a 3-phase AC power line 6B.
Converter 1A (1B) is associated with an automatic margin angle regulator 11A (11B) and an automatic current regulator 13A (13B).
Automatic margin angle regulator 11A (11B) is provided for a prescribed control operation that the margin angle of converter 1A (1B) follows a given margin angle value E17A (E17B). According to this control operation, when value E17A (E17B) is fixed at a constant value, the margin angle of converter 1A (1B) becomes constant. Value E17A (E17B) is obtained from an adder 17A (17B). Adder 17A (17B) receives a minimum margin angle value E18A (E18B) from a margin angle presetter 18A (18B) and an output E48 from an automatic reactive power control circuit 48. The minimum margin angle of converter 1A (1B) is determined by value E18A (E18B). Output E48 is utilized to control the reactive power of AC line 6A (6B).
To achieve the reactive power control, circuit 48 is responsive to an output E46 from a subtracter 46. Subtracter 46 receives at its positive input an output E45 (reactive power reference) from a reactive power presetter 45 and at its negative input an output E47 from a reactive power detector 47. Detector 47 detects the magnitude of reactive power handled by converter 1A (1B). Thus, the reactive power is controlled in response to output E48 which represents the difference between E45 and E47, and the controlled reactive power follows the value of output E45.
In this manner, when the reactive power of AC line 6A (6B) at converter 1A (1B) side is to be controlled, the margin angle of converter 1B (1A) is controlled by output E48 so that the control angle of converter 1A (1B) is changed accordingly.
Incidentally, irrespective of the conversion functions (rectifying, inverting), converter 1A (1B) serves as a phase-delayed load for AC line 6A (6B), and the power factor of converter 1A (1B) is substantially proportional to the cosine of the delay of a control angle.
Automatic current regulator 13A (13B) is provided for a prescribed control operation that the amount of a DC current Id flowing through power transmission lines 3 depends on a given current control value E23A (E23B). Value E23A (E23B) is obtained from a subtracter 23A (23B). Subtracter 23A (23B) receives at its first negative input an output E22A (E22B) from a current/voltage converter 22A (22B), at its second negative input a current margin value E25A (E25B) from a current margin presetter 25A (25B) via a switch 24A (24B), and at its positive input an output E44 from an automatic power control circuit 44. Converter 22A (22B) receives an output E21A (E21B) from a current transformer 21A (21B) arranged at DC line 3, and converts the received E21A (E21B) into output E22A (E22B). Only one of switches 24A and 24B, which allows the corresponding converter (1A or 1B) to operate as an inverter, is closed or turned-on. According to the control operation of regulator 13A (13B), if output E44, which serves as a current reference, is fixed at a constant value, the amount of DC current Id becomes constant. Thus, DC current Id of lines 3 is controlled in accordance with output E44 from circuit 44.
Automatic power control circuit 44 is provided for controlling the power transfer between AC lines 6A and 6B. Circuit 44 is responsive to an output E42 from a subtracter 42. Subtracter 42 receives at its positive input an output E41 (active power reference) from a power presetter 41. The negative input of subtracter 42 receives an output E43 from a power detector 43 which detects the magnitude of power (active power) transmitted through DC lines 3. Thus, the power transfer is controlled in response to output E42 or E44 which represents the difference between E41 and E43, and the controlled power follows the value of output E41.
An output E11A (E11B) from automatic margin angle regulator 11A (11B) and an output E13A (E13B) from automatic current regulator 13A (13B) are supplied to an advanced control angle preference circuit 28A (28B). Circuit 28A (28B) selects either one of the supplied outputs in a manner that the control angle of the selected one is phase-advanced to the control angle of the non-selected one. The selected output from circuit 28A (28B) is converted via a phase control circuit 29A (29B) and pulse amplifier 30A (30B) into gate pulses E30A (E30B) which are used for triggering the switching elements in converter 1A (1B).
It is assumed that switch 24A is in an OFF state while switch 24B is in an ON state. In this case, advanced control angle preference circuit 28A selects output E13A from regulator 13A so that converter 1A serves as a rectifier (AC to DC converter), and circuit 28B selects output E11B from regulator 11B so that converter 1B serves as an inverter (DC to AC converter).
FIG. 2 illustrates a typical characteristic of the FIG. 1 power converter under the above assumption. In FIG. 2, the abscissa indicates a DC current Id flowing through lines 3 and the ordinate indicates a DC voltage Ed applied to lines 3.
Referring to FIG. 2, portions (a), (b) and (c) show an operation curve of converter (rectifier) 1A. Portions (a) and (b) indicate a voltage regulation characteristic. This characteristic depends on the commutation impedance of rectifier 1A as well as other associated circuit impedances thereof. Portions (b) and (c) indicate a constant current characteristic obtained by the operation of automatic current regulator 13A.
Portions (d), (e) and (f) show an operation curve of converter (inverter) 1B. Portions (d) and (e) indicate a constant current characteristic obtained by the operation of automatic current regulator 13B. Portions (e) and (f) indicate a constant margin angle characteristic of inverter 1B. This characteristic is obtained by the operation of automatic margin angle regulator 11B. In FIG. 2, the difference between the DC currents at portions (c) and (d) indicates a current margin defined by value E25B.
The operating point of converters 1A, 1B appears at a cross point (X) in FIG. 2, which is defined by the intersection between the operation curves of rectifier 1A and inverter 1B.
Assume a case wherein margin angle value E17B is increased by the operation of circuit 48 so that phase-delayed reactive power handled by inverter 1B is increased, while both converters 1A, 1B are operated at point (X) in FIG. 2. In this case, DC voltage Ed on lines 3 decreases, and the operation curve of inverter 1B shifts from solid line portions (d), (e) and (f) to broken line portions (dd), (ee) and (ff). Meanwhile, DC current Id from rectifier 1A is increased by the automatic power control operation of circuit 44 so that the decrease in DC voltage Ed (power down) is compensated. Then, the operation curve of rectifier 1A is shifted from solid line portions (a), (b) and (c) to broken line portions (a), (bb) and (cc), and the operating point of converters 1A, 1B is changed from point (X) to (XX).
Since the transmission power can be represented by the product of DC voltage Ed and DC current Id, the curve of constant power becomes hyperbolic and the operating point (X) of converters 1A, 1B is fixed on such a hyperbolic curve HC, as shown in FIG. 2.
As may be seen from the characteristic curves in FIG. 2, when switches 24B and 24A are turned-on and -off, respectively, DC voltage Ed of lines 3 is controlled by inverter 1B while DC current Id thereof is controlled by rectifier 1A, so that the transmission power becomes constant.
In general, a given rated current (which specifies the 100% current output) is assigned to a power converter for safety. From this, although not shown, a current limit circuit is provided at the output stage of power control circuit 44, thereby suppressing the amount of DC current Id below the rated current value.
As mentioned before, when the margin angle varies to control the reactive power, DC current Id of lines 3 varies to control the transmission power. However, if DC current Id increases to exceed the rated current value of the converter (rectifier 1A), the increased DC current is limited at the rated current value of 100% current output. Therefore, when the constant current characteristic of portions (bb) and (cc) in FIG. 2 represents the rated current value and current Id is limited at portions (bb) and (cc), only the margin angle can be increased with the increase of reactive power. In this case, the operation curve of inverter 1B is shifted from portions (dd), (ee) and (ff) to portions (dd), (eee) and (fff), and the operating point of converters 1A, 1B is shifted from point (XX) to (XXX). Since point (XXX) is out of the hyperbolic curve HC of constant power, the transmission power of DC lines 3 becomes low. This is an important problem to be solved.
In the above discussion, the combination of an automatic power control (APC) and automatic reactive power control (AQC) is adapted to the control system of converters 1A and 1B. However, the same discussion may be similarly applied to the combination of an automatic frequency control (AFC) for retaining the system frequency constant and an automatic voltage control (AVC) for adjusting the line voltage constant. In this case, automatic power control circuit 44 and automatic reactive power control circuit 48 in FIG. 1 are replaced with an automatic frequency control circuit and automatic voltage control circuit, respectively, and corresponding signal values used for the system control are changed accordingly.
Assume that the power converter control apparatus is provided with an automatic frequency controller (AFC) and automatic voltage controller (AVC), and that the voltage of AC line 6A is increased for some reason. In this case, for retaining the voltage of AC line 6A constant, the AVC increases the margin angle of inverter 1B so that the reactive power increases. At this time, the amount of the transmission power is reduced by the increase of reactive power, and the frequency of AC line 6A is lowered (or the phase-delay of rectifier 1A is increased) with the increase of reactive power. Then, the amount of DC current Id is increased (i.e., the component of active power is increased) so that the power down of lines 3 is compensated, thereby keeping the frequency constant. However, if DC current Id reaches the rated current value (100% current output), the amount of DC current Id cannot be increased any further, and the AFC operation at the rated current is disenabled. This is another problem to be solved.
In the above description, an automatic current control depending on the automatic power control (APC) or automatic frequency control (AFC) is applied to rectifier 1A side, and an automatic margin angle control depending on the automatic reactive power control (AQC) or automatic voltage control (AVC) is applied to inverter 1B side.
Inversely, an automatic current control depending on APC or AFC may be applied to inverter 1B side, and an automatic margin angle control depending on AQC or AVC may be applied to rectifier 1A side. FIG. 3 shows the operation curve corresponding to FIG. 2, wherein the automatic current control and automatic margin angle control are effected at inverter 1B and rectifier 1A, respectively.
During the operation at point (X) in FIG. 3, when phase-delayed reactive power is increased with the increase of a control delay angle according to the AQC operation, DC voltage Ed is decreased and the operation curve of rectifier 1A is shifted from portions (a), (b) and (c) to portions (aa), (bb) and (cc). Since the APC serves to maintain the transmission power constant, DC current Id is increased by a value corresponding to the decrease in DC voltage Ed. Accordingly, the operation curve of inverter 1B is shifted from portions (d), (e) and (f) to portions (dd), (ee) and (ff). At the end, the operating point (X) is shifted to another point (XX).
Assume here that portions (bb) and (cc) in FIG. 3 represent the rated current value (100% current output) and the control delay angle is further increased by the operation of AQC. Since the amount of DC current Id cannot be increased over the rated value, only the control delay angle is increased. Then, the operation curve of rectifier 1A is shifted from portions (aa), (bb) and (cc) to portions (aaa), (bbb) and (cc) so that the operating point (XX) is shifted to point (XXX), and further APC operation is no longer effected.
As will be understood from the above discussion, even if the automatic current control is effected at inverter 1B side, a similar problem that involved in a case wherein the automatic current control is effected at rectifier 1A side is invited.
The said problem (disenabling of power control at the rated current) could be involved in the following combinations of control modes:
(a) automatic power control (APC) and automatic reactive power control (AQC); PA1 (b) automatic power control (APC) and automatic voltage control (AVC); PA1 (c) automatic frequency control (AFC) and automatic reactive power control (AQC); and PA1 (d) automatic frequency control (AFC) and automatic voltage control (AVC).